1. Field of the Invention
The invention relates to an electric power steering apparatus including an assist motor that assists a steering operation based on a motor rotational angle.
2. Description of the Related Art
As a technology pertaining to an electric power steering apparatus including an assist motor that assists a steering operation based on a motor rotational angle, for example, Japanese Patent Application Publication No. 2006-151360 (JP-A-2006-151360) describes a vehicle steering apparatus. This vehicle steering apparatus computes a virtual steering model input torque based on a steering torque that is detected by a steering torque sensor and an actual steering angle that is detected by a steering angle sensor. Then, an assist motor command value is computed so that the actual steering angle that is detected by the steering angle sensor coincides with a target steering angle that is set based on the virtual steering model input torque and a rack axial force that is detected by a rack axial force sensor.
In order to set an assist motor command value, a rotational angle of a rotor of an assist motor needs to be accurately detected by a rotational angle sensor. However, for example, when a rotational angle sensor is provided in a control unit that controls an assist motor and this control unit is provided at a position on a load side of a motor shaft, which is distant from a rotor of the assist motor, that is, when the rotational angle sensor is provided at such a position that the rotational angle sensor detects a rotational angle of a portion of the motor shaft, which is distant from the rotor of the assist motor, as the motor rotational angle, if a high load is applied and a torsion amount of the motor shaft increases, a detection error in the motor rotational angle that is detected by the rotational angle sensor also increases. Especially when a motor having a small-diameter motor shaft is used, a torsion amount of the motor shaft increases and the above-mentioned detection error further increases. An increase in the detection error causes a problem that an optimum motor current is not supplied to a motor coil and an ineffective current supply increases.